Many processes and devices have been used in the field of pressure sensing. Pressure sensors are generally used and deployed wherever a need for monitoring and responding to pressure changes is necessary. Pressure sensors are commonly used in a variety of automotive, aerospace, commercial, industrial, and consumer applications.
The operational environments in which pressure sensors are required to operate in these applications with high accuracy and repeatability can be very demanding. For example, extreme thermal conditions including thermal shocks in ranges from 160 C to −55 C, exposure to harsh and/or conductive media, withstand high overpressure (proof pressure) cycling without change in calibration and survive high peak (burst) pressures to protect system from potentially catastrophic leaks.
In the case of a pressure sensor that relies upon the use of a pressure transducer (or sense element) consisting of piezoresistive silicon on an etched silicon diaphragm a most cost effective solution for operating in such environments is to use so called “back-side” sensing. With this arrangement the only parts of sensor which are exposed to the media are the electrically isolated cavity side of the pressure transducer, the adhesive used to bond the transducer die to a substrate and finally the substrate itself.
FIG. 1 illustrates a cross sectional view of a prior art direct chip-on-board mounted pressure sensor apparatus 100. The configuration illustrated in FIG. 1 generally includes a pressure transducer/die 102 that is attached to a printed circuit board (PCB) substrate 106 with the assistance of a die-attach adhesive 104. The bottom portion of the pressure sensor apparatus 100 includes a housing 108. A top cover 105 or protective portion is also included. The configuration depicted in FIG. 1 is generally presented for background and edification purposes only and is not considered a limiting feature of the disclosed embodiments.
For such a design to operate reliably under the conditions described above typically requires the use of attach materials which have high strength and chemical resistance. The rigid mounting of stress sensitive die (e.g., pressure transducer or pressure sense element) onto a substrate where there is large mismatch in thermal expansion coefficient between the die and the substrate can introduce high levels of package stress, which can result in output errors, non-repeatability and potentially, mechanical damage.
Pressure sensor apparatus 100 depicted in FIG. 1 is an example of a device in which a large mismatch in the thermal expansion coefficient between the die and substrate can exist. Also under extreme operating conditions stress can be transferred into the substrate from outer packaging such as encapsulation. These stresses can be transferred into the die attach adhesive and the pressure transducer itself. The resulting high levels of stress at or near the pressure transducer can cause non-repeatability errors due to effects such as creep, larger thermal errors (for example temperature coefficient of offset) and even destructive levels of mechanical stress. It is therefore believed that a solution to these problems lies in the implementation of an improved pressure sensors method and apparatus, which is disclosed in greater detail herein.